Not Applicable.
Not Applicable.
The present invention relates to a method and apparatus for refreshing a display that uses an internally controlled backlight. More particularly, the present invention is directed to a passive liquid crystal display with light source that is used in a front, edge or backlight configuration that has a wide dimming range while minimizing display artifacts caused by beating of display refresh and backlight signals.
Conventional liquid crystal displays have been known in the art for displaying information in a variety of applications. These include electronic instrumentation, calculator displays, and the like. Avionics instrumentation frequently uses a variety of LCD displays to provide information in the cockpit. In certain scenarios, especially in dark conditions outside the cockpit, LCD displays become overly bright and the pilot wishes to dim the LCD displays to allow his/her eyes to adjust to the outside conditions.
LCD displays utilize non-emitting pixels (liquid crystals) and therefore require a light source. Some LCDs are of a reflecting type and use either an external source such as the sun, or an internal front lighting source. Some LCDs are transmissive only and require an internal light source. Some LCDs are trans-reflective which use both reflective and transmissive modes. Possible light sources include cold cathode flourescent tubes, hot cathode flourescent tubes, LEDs, and Electroluminescents.
To provide dimming of the display, the light source intensity and/or light source xe2x80x9con timexe2x80x9d can be controlled. When the intensity method is used, typically, the amount of current used in the light source is varied. Typically this method does not yield a large dimming ratio because of the non-linear relationship of current to light and the difficulty for some technologies to know when light is being produced. Light source xe2x80x9con timexe2x80x9d controls brightness by only having the display on a certain amount of time for each backlight period. This method will be referred to as the duty cycle method. Typically the backlight period is greater than 60 Hz. The aspects and features of the present invention are most useful with the duty cycle method of dimming. Note that this flashing on and off of the light source typically allows much wider dimming ranges and also causes the source to behave like a strobe light.
Passive LCD pixels begin relaxing immediately after being refreshed. A duty cycle light source can cause this relaxation effect to be seen as slightly discolored color bands on the display. This occurs much the same way a strobe light appears to cause a moving object to be non-moving. Because liquid crystals are formulated from a specially manufactured fluid or oil, the extent and speed of relaxing increases as the temperature is increased because the liquid viscosity is decreased. The display aliases become more pronounced as the liquid crystals heat up during normal use or when used in high temperature environments and the oil inside them becomes less viscous. The relationship between the light source and the display frequency can cause various artifacts. The artifacts include fixed color bars or fast or slowing moving bars. Moving colors bars can be caused by unlocked frequencies or locked frequencies where the relationship of frequencies causes the color bars to appear in a fixed pattern that is much lower than 60 Hz. For the purposes of this discussion, xe2x80x9cunlockedxe2x80x9d means that the frequencies can vary independently. xe2x80x9cLockedxe2x80x9d means the ratio of the frequencies stays constant even as they drift. The stated artifacts are caused by light sources that are used in duty cycle mode whether they are used in front light, backlight, edge light or any other mode. It should be noted that in the case of unlocked frequencies, either the light source or the display may not show problems at certain times and at other times, have very visible aritfacts. This suggests that a fixed frequency ratio is generally desirable.
A conventional solution to minimize the unwanted display aliases is increasing the display refresh frequency, so that the liquid crystals do not have enough time to completely degrade and so that the difference in color from the center of the color band to a point farthest away is much smaller. This method can greatly reduce the color bands. Care must be taken, as discussed below, when controlling the backlight frequency to minimize certain artifacts and to avoid others.
Another known solution to reduce visible display aliases in LCD displays is to increase the light source frequency while keeping the display refresh frequency at its original level. This reduces the display aliases on the display, but generally has the undesired consequence of reducing the backlight dimming ratio for light sources that have a minimum xe2x80x9con time.xe2x80x9d For example, doubling the backlight frequency cuts the available dimming ratio in half. This method reduces the visibility of the color bands by causing them to be in different parts of the display, thus, averaging the bands over the entire display.
Traditionally, backlights had limited dimming capabilities, resulting in dimming ratios of 20 to 1 or less. Recently, industry sought a need for greater dimming for these passive displays. As a result, methods were developed for creating greater dimming ratios for adjusting the illumination of backlights in passive LCD displays. In particular, a period of time is established during which the backlight may be turned on. This period is set long enough to provide a wide range of dimming. Dimming is accomplished when the backlight is on for a time less than the full period available. Dimming of backlights have improved to ratios greater than 500 to 1.
Although the prior art includes various methods and apparatuses that refresh liquid crystal displays, there exists a need for a liquid crystal display having a wide dimming range while reducing the effects of display aliases and beat frequency artifacts. The present invention fills these and other needs, and overcomes the short-comings of the prior art.
It is an object of the invention to provide a method and apparatus for refreshing a liquid crystal display that allows for dimming of the display so that display aliases are reduced on the display.
It is also an object of this invention to provide a method and apparatus for refreshing a liquid crystal display that allows for dimming of the display so that visible beat frequency artifacts are minimized on the display.
To accomplish these and other related objects, the invention is directed to a method and apparatus for displaying data on a liquid crystal display. The apparatus consists of a central processing unit, a LCD display with an associated LCD controller, and a light source with an associated light source controller. Displaying data is accomplished by generating a backlight frequency via the backlight controller to a period long enough to allow for dimming of the display at ratios greater than 20 to 1. The LCD controller sends a display refresh frequency for reenergizing the pixels (liquid crystals) in the display. In accordance with the invention, the display refresh frequency and backlight frequency are set such that the ratio between them includes a fraction. Particularly, the stated ratio is preferably 6.5.